That's simple. It's because humans are passionate animals. They might have a family member or close friend who lost everything in a record breaking typhoon, like Katrina. So, when someone says it's just BS, they feel like punching them. That knife cuts both ways. Skeptics might have relatives who are right now losing their fracking jobs, so they feel passionate too. It's natural. Everyone has a brain and likes to use it to decide for themselves what is right or wrong, and there is both good science and talking idiot media heads on both sides. I think the reason that the passion runs higher on the environmentalist side is that the perception of downside loss is much greater. It's not just losing their job or being unnecessarily inconvenienced.
A good example that skeptics should take a serious look at is the ozone problem in the 70's and 80s. The net result was that government required everyone in the world to get rid of CFC's to stop the ozone hole that formed over antarctica. There was a lot of controversy about it at the time, and when the law got passed everybody had to switch freon, the cars got a little more expensive, and for the first few years the AC didn't work fer shit. Somehow, we got over it, made the transition, and then sure enuf - the ozone hole started to close in 2006 just as the scientists had predicted it would.
Think about that - what if the skeptic community was a big then as it is now. What if the CFC's didn't get eliminated and people had to wait to see a real impact before they would change the refrigerant in their car? That woulda been a pretty bad scenario because it might have killed most of the plants on earth if we'd let it continue for another 30 years, and then it woulda been too late.
Anyway that explains why the so called "cult" members get so passionate. But it doesn't explain why skeptics are so passionate about the CO2. One explanation is there isn't a good priced solution - but if think about that compared to the potential down side, then you might wanna think instead about whether it's really going to hurt you.